Obsessive-compulsive disorder (OCD) is a chronic and debilitating illness affecting up to 2-3% of the population. The only medications proven effective for OCD are serotonin reuptake inhibitors (SRIs), but even with SRI treatment, most patients continue to experience significant OCD symptoms, impaired functioning, and diminished quality of life. With the goal of maximizing treatment outcome, the primary aim of this competing continuation/collaborative R01 application is to compare the two SRI augmentation strategies in OCD proven effective in randomized controlled trials: the addition of the antipsychotic medication risperidone (RIS) and the addition of cognitive-behavioral therapy consisting of exposure and ritual prevention (EX/RP). Because prior studies found that even with 6-8 weeks of antipsychotic or EX/RP augmentation, most patients have significant residual OCD symptoms, the second aim is to examine whether patients who benefit from 8 weeks of augmentation will maintain their gains or improve further with continued treatment. Accordingly, during the Acute Augmentation Phase, 100 patients with OCD (50 per site) who have received an adequate SRI trial but still experience clinically significant symptoms will remain on their SRI and be randomized to 8 weeks of one of three adjunctive treatments: 1) twice-weekly EX/RP;2) RIS;or 3) pill placebo. Patients who benefit from the Acute Augmentation Phase will enter the Continuation Phase, and receive the same adjunctive treatment for an additional 24 weeks. This study will be the first to compare the only SRI augmentation strategies for OCD with strong empirical support and to determine whether OCD patients can achieve and maintain an excellent treatment outcome if provided up to 32 weeks of adjunctive treatment. Relevance: Given that most patients who take SRIs for OCD continue to experience significant OCD symptoms, determining how best to augment SRI response and help OCD patients achieve an excellent treatment outcome is an important public health challenge. The long-term goal of the proposed study is to provide clinicians with specific guidelines for how to maximize treatment outcome and help people with OCD lead productive and high quality lives.